1. Field of the Invention
The present invention relates to an optical pickup device or an optical head device.
2. Description of Related Art
As optical information recording mediums, various kinds of optical disks, called Laser Disk (LD), Compact Disk (CD) and Digital Video Disc (DVD), have been known. In such optical disks, standardizations are made for the respective items such as thickness of the substrate in different specifications. Moreover, in optical disks of multi-layered structure included in a DVD specification, an effective thickness of the optical disk is different between the recording layers. The most suitable numerical aperture (NA) of a reading objective lens is differently determined among the different optical disks.
Compatible players capable of reading out recording information from, for example, both CD and DVD are required. There are the following differences between optical systems for reading out the recorded information on the CD and the DVD.
(1) Difference in the numerical apertures NA: the numerical aperture for the CD is set to be 0.45, while the numerical aperture for the DVD is set to be 0.6.
(2) Difference in the substrate thicknesses each from a recording surface (reflection surface) to an optical disk surface: the substrate thickness for the CD is set to be 1.2 mm, while 0.6 mm for the DVD.
(3) Difference in wavelengths of optimal reading-out lights: the wavelength thereof for the CD is set to be 780 nm, while 650 nm for the DVD.
Consequently, these differences must be overcome so as to realize an optical pickup device for the CD/DVD compatible player.
To achieve such compatible players, it is often thought that an objective lens having two focal points should advantageously be employed. This is because an objective lens having a single focal point has a numerical aperture most suitable for one optical disk, the numerical aperture being improper for the other optical disk. For the other optical disk, such objective lens produces an aberration such as a spherical aberration.
An example in which the two focal point pickup is realized using a hologram lens has been disclosed in Japanese Patent Application Kokai No. 7-98431/1995. This two focal point pickup forms a complex objective lens composed of an objective lens 1 and a hologram lens 2, as shown in FIGS. 1A and 1B. On the hologram 2, a plurality of diffraction grooves 3 are concentrically formed over the region corresponding to the numerical aperture for the CD. A light beam is diffracted by these diffraction grooves 3, so that it is divided into a zero-order diffraction light and a first-order diffraction light. Thus, respective incident angles of them onto the objective lens 1 are made different, whereby each focal point is formed on corresponding one of recording surfaces of the optical disks. At this time, the light beam is allowed to transmit through the region where no diffraction groove 3 is formed, without being diffracted, and this transmitted light is converged by the objective lens 1 together with the zero-order diffraction light. As a result, the transmitted light, the zero-order diffraction light, and the first-order diffraction light will have different numerical apertures, respectively. As shown in FIG. 1A, the first-order diffraction light diffracted by the diffraction grooves 3 is employed for reading a CD 4 of a small numerical aperture. As shown in FIG. 1B, the transmitted light and zero-order diffraction light of larger numerical aperture are employed for reading a DVD 5.
The structure principally involves the formation of reading spots with a single light source. A light source emitting a reading light of a wavelength of 650 nm, which is most suitable for the DVD, is normally used. However, a light most suitable for reading out a recordable/reproducible recording medium has a wavelength of 780 nm, which is called a CD-R (CD Recordable or a R-CD: Recordable CD) belonging to the CD group. Namely, the light having the wavelength as great as 650 nm is not suitable for reading out information recorded on the CD-R. Therefore, in order to realize a compatible player capable of performing a good recording/reproducing of information for a disk such as the CD-R, the optical pickup or head is preferably constructed with light sources of a plurality of wavelengths but not a single light source, each light source being suitable for corresponding one of the disks. However, such an optical system composed of a plurality of light sources typically has the disadvantage that the structure of the optical pickup or the whole head tends to be complicated and thus be larger in size.